Optical scanning device for rapid spectroscopy



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. NOV. 3, M CLERC OPTICAL SCANNING DEVICE FOR RAPID SPECTROSCOPY Filed NOV. 29, 1967 2 Sheets-Sheet 1 .Beamers l f on :Maase/51 NOV. 3, 1970 M, CLERC I 3,537,795

OPTICAL SCANNING DEVICE PoE RAPID SPECTROSCOPY Filed Nov. 29, 1967 2 Sheets-Sheet a /NVE/V TOR M/CHEL a Eea TTR/VEY or 95% of the time. Thus certain chemical species of short United States Patent 3,537,795 OPTICAL SCANNING DEVICE FOR RAPID SPECTROSCOPY Michel Clerc, Gif-sur-Yvette, France, assignor to Commissariat a lEnergie Atomique, Paris, France Filed Nov. 29, 1967, Ser. No. 686,666 Claims priority, applicaton France, Nov. 30, 1966, 8 700 Int. Cl. G01n 21/34; G01j 3/06 U.S. Cl. 356-51 7 Claims 1 ABSTRACT OF THE DISCLOSURE The present invention relates to optical scanning devices, that is to say to devices which direct a succession of luminous beams spaced apart in time onto a single optical detector or a small number of optil detectors. The invention is concerned more particularly, but not exclusively, with devices intended for rapid infra-red spectrography, that is to say devices which permit the recording of a series of infra-red spectra, spaced apart in time, in the course of a single revolution of the mirror for scanning the wavelengths in infra-red spectrograph or spectrophotograph.

Scanning devices for infra-red spectrography are already known, which can, for example, follow the evolution, in the course of time, of the concentration ofxtransitory chemical species appearing in a rapid chemical reaction. Most of these devices use a rotating mirror which is a plane mirror placed in the path of the beam to be observed which is dispersed by a diffraction grating or a prism, In particular, the rotation of the mirror reflecting the beam has been effected in a Littrow mounting khaving a double path for the luminous beam through the dispersing system (notably a prism). This method has the disadvantage that the periods of useful scanning are separated by periods, of the order of twenty times longer, during which the beam is not reflected by the mirror and thus is not recorded. Thus, if it is supposed that a mirror is used which reflects on its two faces and that the angle of rotation during which each face of the mirror reflects the desired wavelengths is equal to 9 (it is in general between and 10), the relation between the durations of the alternating inactive and active periods is equal to iso-9:19

Thus there is a risk that some very interesting transitory phenomena can be'missed, since the device is blind 19/ 20 lifespan risk to be not detected. It will be noted that increasing the speed of rotation of the mirror, although it permits the absolute duration of the inactive periods to be reduced, does not modify the r/atio mentioned abovq 7 between the inactive and active periods. One might think of using, instead of a single mirror, a system of mirrors,

disposed to form a rotating prism, -but it is more difficult tofmake such a mounting turn at high speed. The only solution with devices of the prior type, if one wishes to be sure that certain very short phenomena do not escape, is to use several spectrographs, but this is a costly solution.

An object of the present invention is to provide an optical scanning device which does not have the disadvantages mentioned above.

Another object of the invention is to improve such devices, in particular so that they insure, on the one hand, a high ratio between the duration of each usable operational period and the duration of each unusable nonoperational period, and a reduction of the duration of each non-operational period, and on the other hand, a reduced cost price and anincreased robustness.

According to the present invention, an optical scanning p 1 device is characterized by the fact that it comprises, in

combination with the usual rotating mirror (called hereafter first mirror)A receiving the dispersed beam to be observed, a series of Xed mirrors (called hereafter second mirrors), advantageously parabolic or elliptical, disposed to receive succesively, in the course of a halfrevolution of the first mirror, the dispersed beam refiected by the first mirror and to focus these beams on one, two or a small number of slits, these slits being projected by a small number of concentration mirrors (called hereafter third mirrors), adavntageously elliptical, on one, two or a very small number of detectors, there being fewer third mirrors than second mirrors.

The invention is particularly applicable to optical scanning devices for rapid infra-red spectrography.

IParticular embodiments of the invention will now be described by way of example, with reference to the`accompanying drawings in which:

FIGS. l and 2 show two embodiments of an optical scanning device, provided with the improvements according to the invention. 'I

These two embodiments are particularly intended for rapid infra-red spectrography.

In the embodiment illustrated in FIG. 1, the rotating (first) mirror has been represented at m, this rotating mirror having two reflecting faces m1 and m2 which receive the dispersed beam R2 to be observed. In this embodi' ment, the initial parallel beam R1, is dispersed by the prism 1, the cross-section of the beam 'R1 leaving the prism (the extreme luminous rays of the beam have been shown for the two limit wavelengths) being reduced by a Cassegrain telescope havingmirrors 2 and 3 (or pos'sibly by a telescope having two concave parabolic mirrors of the type used in the rotating mirror Littrow mountings) in order` to be able to use a first mirror m of small dimensions, and hence adapted to be driven at very high speeds of rotation (for example of the order of 5,000 to 10,000 revolutions/ second) The device comprises in addition a series of fixed l(second) mirrors, for example two pairs of parabolic mirrors M1, M2 and M3, M4, the mirrors M1 and M2 of the first pair having a common focus F1, whereas the mirrors M3 and M1 of the second pair have a common focus F2 symmetrical with respect to F1 about the axis xy of rotation of the rotating first mirror m, this axis being projected at 4 in the plane of the figure.

In the position of the first mirror m illustrated in FIG. 1, the reflecting face m1 of this mirror reects the beam R2, as a reected beam R3, onto the second mirror M1 which retects the beam R3, as a reflected beam R4, thus forming a ray on the focal surfaces f1 passing through F1. In FIG. 1, the path of the two luminous beams of extreme wavelengths l1 and l2 forming images at F1 and F3 have been shown, each Wavelength arriving at the mirrors Patented Nov. 3, 1970 in the form of a parallel beam, but' at an angle of inci-1 dence which is'a function of this wavelength.

A slit G1, formed in a wall portion, is placed at F1 perpendicular to the beam R4, and the rays of different wavelengths pass successively through this slit when the first mirror m rotates through an angle, of the order of 20 for example, in the course of which it reflects, with its face m1, the beam R2 onto the second mirror M1.

If it is supposed that the first mirror m rotates in the direction of the `arrow T, its face m1, after reliecting the beam R2 onto the fixed second mirror M3, reiiects this beam successively onto the fixed second mirrors M1, M1 and M2. The mirror M2, like the mirror M1, reects the beam R2 onto the slit G1 disposed at F1, whereas the mirrors M3 and M1 reect the beam R2 onto a slit G2, also formed in a wall portion, disposed at F2 perpendicular to the incident beam (forthe mirrors M2, M3 and M1 only the median ray has been shown in FIG. 1). A casing 5 limits the luminous beam arriving at the mirror m or reiiected by this mirror.

After a half-revolution of the mirror m, this mirror is once again in its initial position, except that the faces m1 and m2 are interchanged, and an analogous180 cycle begins again. Thus the face m2 refiects the beam R2 successively onto M3, M4, M1 and M2. The two latter mirrors reflect the rays successively through the slit G1 placed at F1, whereas the mirrors M3 and M1 reflect the luminous beams to make the rays pass successively through the slit G2 placed at F2.

The elliptical (third) mirrors N1 and N2, acting as camera objectives, focus the light from each slit G1 and G2 respectively at D1 and D2 where two rapid detectors (for example of the ldoped germanium type) are placed. These detectorsD1and D2 transform the luminous intensity (infra-red) into an electric voltage in a substantially linear manner, the voltages of the two detectors being indicated by a single cathode-ray oscillograph which records successively the spectra to be studied.

In one complete revolution of the mirror m, the beam R2is reiiected successivelylas indicated in the following table:

mz-Ma-Fz-Ns-Dz mz-Mi-Fz-Nz-Dz Successive indication by a single cathoderay oscillograph.

This ratio can be increased by disposing one or more pairs of supplementary fixed second mirrors within the limits of the available space. (Another solution is given by the embodiment of FIG. 2.)

One of the advantages vof the mounting of FIG. 1 is its simplicity and the fact that all the fixed parabolic second mirrors are of identical form.

In a variation of the embodiment shown in FIG. 1, the four fixed second vmirrors M1, M2, M3 and M11 are not parabolic mirrors, but elliptical mirrors disposed so that these four elliptical mirrors have a first common focus at 4, at the intersection of the plane of FIG. 1 with the axis of rotation xy of the rotating first mirror m, and a second focus at F1 for the mirrors M1 and M2 and at F2 for the mirrors M3 and M4. In this case, any luminous ray coming from 4 will pass through F1 or F2, all the optical paths 4M1F1, 4M2F1, 4M3F2, 4M4F2 being equal, according to the fundamental property of ellipsoids. To obtain the best astigmatic conditions, the apparatus is arranged so that the luminous spot formed by the beam R2 on the mirror m is as small as possible, for example by defocusing the Cassegrain telescope 2 and 3 in order to make the beam R2 converge on the surface m1 or m2 of the rotating mirror, in the neighbourhood of the point 4. Apart from these two modifications (mirrors M1 to M4 elliptical and defocusing of the Cassegrain telescope), the mounting is similar to that illustrated in FIG. 1.

The embodiment illustrated in FIG. 2 represents, in a way, the extension in space of the embodiment of FIG. 1.

The fixed parabolic second mirrors M1 to M3 are disposed about the vertical axis xy of rotation of the plane first mirror m having reecting faces m1 and m2, in the neighbourhood of the horizontal plane P in which is located the central ray of the dispersed beam R2 to be observed. (FIG. 2 shows at p thecircle of the plane P passing through the reecting surface of the mirrors M1 to M3 disposed between the circles p1 and p2, these mirrors M1 to M2 occupying substantially half the area of of the band or zone comprised between p1 and p2.) The foci F1 to F8 of these parabolic mirrors M1 to M2 are disposed in a horizontal circle c, of small diameter, placed well above the zone p1-p2 and centered on the axis xy.

In FIG. 2, the path of the middle ray of the beams R2, R3 and R4 have been shown. At the point F1 is disposed a slit G1, at the point F2 a slit G2 etc. slits G1 to G2 placed at the foci F1 to F2 of the fixed parabolic mirrors M1 to M8 are disposed in a conical wall C (between two circles c1 and c2) tangent to the circle c and perpendicular to the middle rays (such as R4) coming from the mirrors M1 to M8.

The two elliptical third mirrors N1 and N2 reiiect the image of the slits G1 to G11-through each of which pass successively the successive wavelengths of the spectrum to be examined when the mirror m rotates-at points D1 and D2 where ultra rapid infra-red detectors are placed (for example of the doped germanium type) which transform the luminous intensity into an electric voltage in a substantially linear manner, the output of these detectors being connected for example to a single oscillograph.

In FIG. 2, there is shown Vin dashed lines, about the middle ray, the cross-section of the various beams (circular for the beams R2 and R3, elliptical for the beam R1), and by arrows the direction of propagation of this middle ray right up to the detector placed at D1.

In the embodiment of FIG. 2, each face of the mirror m permits, in the course of a half-revolution, the recording of eight spectra. Thus 16 spectra are obtained per revolution of the mirror, the ratio between the durations of the inactive periods and the active periods being of the order of 1, thus a coefficient of use of 50%, which is excellent.

The present invention thus provides, regardless of the embodiment adopted, an optical scanning device, in particular for rapid infra-red spectrography, which has, with respect to existing devices, numerous advantages, in particular the following:

First of all it permits the recording of a large number of spectra per revolution of the rotating mirror, the proportion of the useful periods for recording the spectra in the total duration being high.

It comprises only a single rotating element, namely a plane mirror, which can be of small size and thus driven at very high speed.

The complications of mounting are carried over to the fixed elements of the device, which simplifies adjustment.

The device is robust and of a reduced cost price.

Various modifications are possible, without departing from the scope of the invention, such as using, instead of elliptical or parabolic mirrors, plane mirrors disposed to reflect towards a limited number of points (such as F1 and F2) the beams (such as R3) refiected by the rotating mirror m.

Although the invention has been described with reference to particular embodiments, it should be understood The eight that the invention is not limited thereto, and that various modifications are possible without departing from the spirit or scope of the invention.

What I claim is:

1. Optical scanning device comprising: means for dispersing a beam to be observed to form a dispersed beam comprising rays of different wave lengths, a first plane mirror, with two parallel reecting faces, rotatably mounted for reflecting said dispersed beam, a plurality of substantially identical fixed second mirrors disposed to receive successively, in the course of a half-revolution of said first mirror, the beam reccted by said first mirror, a plurality of wall portions each containing a slit, each said second mirror serving to focus the beam which it receives onto one of said slits whereby said rays of different wave lengths pass successively through said slits, at least two third mirrors disposed behind said slits to reflect said rays passing through said slits, there being fewer third mirrors than second mirrors, and at least two detectors, but not more detectors than third mirrors, disposed to receive the rays reflected by said third mirrors. 2. Optical scanning device according to claim 1 in which said second mirrors are parabolic.

3. Optical scanning device according to claim 1 in which said second mirrors are elliptical.

which said third mirrors are elliptical.

5. Optical scanning device according to claim 1 in which said second mirrors are parabolic and are disposed in two groups, the second mirrors of one of said two groups having a common focus and the second mirrors of the other of said two groups having a common focus, said two foci being symmetrical with each other about the axis of rotation of said rotatable first mirror,

and in which two wall portions are provided, each having a slit, one of said wall portions being disposed with its slit at one of said foci, and the other of said Wall portions being disposed with its slit at the other of said foci,

and in which there are only two third mirrors, said third mirrors being disposed respectively behind said two slits.

6. Optical scanning device according to claim 1 in Iwhich said second mirrors are parabolic and are disposed substantially in a circle about the axis of rotation of said rotatable first mirror, said second mirrors having their foci disposed in a circle of smaller diameter than the circle in which said second mirrors themselves are disposed, said circle of foci lying in a plane perpendicular to the axis of rotation of said rotatable first mirror, said plane of foci being separated in the direction of said axis of rotation from said circle of second mirrors,

and in which there are a plurality of wall portions,

each having a slit, each slit corresponding to a second mirror, said wall portions being disposed with their slits respectively at said foci to receive substantially perpendicularly the beam refiected by said second mirrors.

7. Optical scanning device according to claim 1 in which there are two third mirrors, said third mirrors being elliptical camera objectives,

and in which there are two detectors, said detectors being doped germanium detectors.

U.S. Cl. X.R. 

